There are various types of electric motors (referred simply to as “motors,” including generators). In recent years, synchronous motors are focused on, with which electric power saving, high efficiency, high torque or high output can be expected, in accordance with the development of inverter control and the spread of rare-earth magnets having high magnetic characteristics.
The synchronous motor is a motor that may comprise permanent magnets for the rotor and an armature coil for the stator, i.e. an AC motor in which the armature coil is supplied with an alternate current (AC) to generate a rotating magnetic field around the stator, thereby driving the rotor. Synchronous motors are classified into surface permanent magnet synchronous motors (referred simply to as “SPM motors”) in which the permanent magnets are disposed on the surface of the rotor and interior permanent magnet synchronous motors (referred simply to as “IPM motors”) in which the permanent magnets are disposed inside the rotor. Nowadays, the IPM motors are becoming mainstreams because the enhanced torque and electric power saving can be achieved owing to the magnet torque and the reluctance torque in accordance with the salient pole ratio and the reliability can also be improved since the permanent magnets are prevented from flying away.
For conventional IPM motors, an interior permanent magnet-type inner rotor has been used, in which the magnetic poles are made by inserting sintered rare-earth magnets into slots of the rotor core. The sintered rare-earth magnets are obtained, such as by cutting and polishing the magnets into predetermined dimensions. However, despite a small degree of freedom in forming a sintered magnet, the cross-sectional shape of a magnet with consideration for the optimized design may often be approximately a circular arc shape or an elliptical shape. In addition, the radius of curvature of the inner circumferential side surface and that of the outer circumferential side surface may be different and the thickness of magnet may vary in the circumferential direction. Such processing of magnets is difficult and the use of sintered magnets may thus lead to high cost. Moreover, defects and the like are likely to occur when the magnets are inserted into the slots. Accordingly, it has been compelled to use sintered magnets of a simple shape such as a plate-like shape in many IPM motors. In such circumstances, Patent Literature 1 below proposes an interior permanent magnet-type inner rotor (which may be referred to as an “IPM inner rotor” or referred simply to as an “inner rotor”) having magnetic poles of anisotropic rare-earth bond magnets that are molded by injecting a flowable mixture of anisotropic rare-earth magnet power and a binder resin into the slots in oriented magnetic fields. Note, however, that Patent Literature 1 merely describes a single body of the inner rotor.